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The Moon is Earth's only natural satellite, orbiting at an average distance of 384 399 km (238,854 mi; 30 Earths across).It faces Earth always with the same side.This is a result of Earth's gravitational pull having synchronized the Moon's rotation period with its orbital period (lunar month) of 29.5 Earth days.
By observing the duration of an eclipse, which is about 4 hours, and comparing that to the orbital period of the moon (28 days), the circumference of the moon's orbit was determined. [38] Later, in 129 BC, Hipparchus performed a calculation based on observing a solar eclipse from two separate locations. In one location, the eclipse was complete ...
The Moon's elongation is its angular distance east of the Sun at any time. At new moon, it is zero and the Moon is said to be in conjunction. At full moon, the elongation is 180° and it is said to be in opposition. In both cases, the Moon is in syzygy, that is, the Sun, Moon and Earth are nearly aligned.
Parts-per-million chart of the relative mass distribution of the Solar System, each cubelet denoting 2 × 10 24 kg. This article includes a list of the most massive known objects of the Solar System and partial lists of smaller objects by observed mean radius.
Earth and the Moon as seen from Mars by the Mars Reconnaissance Orbiter. The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter about one-quarter of Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto.
With a diameter of about 5,270 kilometres (3,270 mi) and a mass of 1.48 × 10 20 tonnes (1.48 × 10 23 kg; 3.26 × 10 23 lb), Ganymede is the largest and most massive moon in the Solar System. [45] It is slightly more massive than the second most massive moon, Saturn's satellite Titan, and is more than
Triton was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. The 1989 flyby of Triton by the Voyager 2 spacecraft remains the only up-close visit to the moon as of 2025. As the probe was only able to study about 40% of the moon's surface, multiple concept missions have been developed to ...
Size comparison of Europa (lower left) with the Moon (top left) and Earth (right) Europa is slightly smaller than the Earth's moon. At just over 3,100 kilometres (1,900 mi) in diameter, it is the sixth-largest moon and fifteenth-largest object in the Solar System.